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How To Choose The Correct Speaker?

by admin on February 6, 2015

Are you wanting to build a subwoofer? A high output mid bass cabinet? How about a full range PA box? There are specific low frequency drivers for each of these needs. A speaker that is best for a subwoofer box would be a poor choice for a mid bass application. A full range box will require a completely different speaker. Once you understand how to look at parameter values, you can use them to find the best component for your own particular need.

The first parameter to look at is the Qts.
This particular parameter will show which category the speaker will be best for. Remember these are guidelines, and there are always exceptions. Usually you can go a little either way with your Qts values for each of these categories.

.40 and above. As you go higher in value above .40, your speaker will only have flat response in very large vented boxes or sealed boxes. .40 to .50 will usually work well in a sealed, vented band pass box. That is an enclosure with a sealed rear chamber and a vented front chamber. Usually the .45 to .55 area is best for this type of box.

.37 to .40. This category works well for larger subwoofer boxes where the box can go as low as possible, and it won’t be used above 80 to 100 Hz. These woofers tend to lack punchy mid bass detail but still respond quick enough in the low bass region.

.30 to .36. This category would be the best choice for a high output subwoofer where the box can reproduce low frequencies without having to use a lot of equalization. This is a good region for the bass speaker of a full range box. Band pass enclosures with vented chambers on either side of the driver like Qts values in this area. Values in .35 area will perform well in larger boxes and values in the .27 to .30 area will be best for smaller band pass boxes with a slightly higher low frequency limit.

.26 to .29. This range is ideal where box size is critical and you still desire to get flat response to your tuning frequency. If you choose a woofer with a Qts of lower than .27, added equalization will be required to get flat response to your tuning frequency. This is a good area for small, full range boxes and small subs.

.22 to .26. Speakers in this area are ideal for dedicated mid bass speakers and small bass cabinets where added equalization is required. There is nothing wrong with this approach. The lower the Qts, the quicker the cone can move. With some careful equalization, you can end up with a stunning subwoofer in a smaller box size.

.21 and below. This range would work well for a horn loaded mid bass speaker. It would also be good for a mid bass component in a direct radiating full range box. You will have hard time getting low bass out of any speaker with a Qts below the .20 area.

Whatever the value, Qts should not be the only parameter to consider in your box design. An understanding of how Fs [free air resonance] and Vas [volume of air that matches the suspension springiness of the cone assembly] will be required. Understanding how these figures interrelate will help in determining speaker selection. Below is an optimum box design chart.

Qts of your speaker

Multiply your Fs
by this number to
get the frequency
to tune your box at

Divide this number
into the Vas to
get the size to make
your box

Multiply this number by
your Fs to tell the -3db
down pint of your box

.21

1.84

6.95

2.39

.23

1.69

5.61

2.16

.25

1.56

4.58

1.96

.27

1.45

3.77

1.79

.29

1.35

3.12

1.64

.31

1.27

2.59

1.50

.33

1.20

2.15

1.38

3.5

1.13

1.79

1.27

.37

1.08

1.49

1.16

.39

1.03

1.23

1.06

.41

.98

1.00

.97

The above chart gives an indication of how all these key parameters relate with each other. By using this chart, the absolute optimum box can be identified for the maximum power handling of the speaker selected. To maximize the output of any speaker, it is vital to match the tuning frequency and the optimum box volume. This will vary considerably from speaker to speaker.

Low frequency speakers can be easily damaged at high power without proper tuning of the enclosure. A low frequency speaker handles its most power at the frequency the box is tuned at. It is most easily damaged if it is allowed to play at high power levels that are more than 10 to 15% below that frequency. It is also susceptible to being over driven in the region that is slightly higher than the tuning frequency. This region is known as the minimum impedance area. At high levels, your amplifier is able to overheat or overdrive the speaker in this region also.

By carefully choosing the correct tuning point for the speaker chosen, maximum power handling will be achieved throughout the frequency range of the box. Tuning too high will allow the speaker to be easily overdriven below the tuning point. Tuning too low will create excess cone movement above the tuning point, significantly reducing performance of the box.

Here are a few examples.

• By using the design chart, a speaker with a Qts of .4 determines that your optimum box volume is equal to the Vas parameter. Using this is a starting indicator of the box size and the motor strength of the speaker.

• A speaker with a Qts of .34 will determine a box volume that drops to one half of the Vas parameter.

• At .30, it will perform in a box volume of one third theVas parameter.

• In the .26 area it drops to one fourth of the Vas value. This is because as the Qts goes lower, the magnet/voice coil strength has the ability to compress air linearly in smaller and smaller boxes.

The Qts to Vas ratio should be determined before moving forward with the box design. When using the optimum box chart, it becomes increasingly difficult to get a desirable box as the Qts goes below .33 to .35. In the .25 range, the box volume gets so small that it will need to be tuned too high. Remember, it is very important to maintain a proper tuning ratio if the box size is changed.

The next parameter to consider is the Fs.
The lower the Fs value, the lower in frequency any box size designed will respond to. After designing a few boxes, the relationship of these parameters becomes clearer. By looking at individual speaker parameters, it can then be determined what a particular speaker was designed for. This will guarantee that the correct speaker is chosen for the specific box and application.

It is possible to go up to 2 times the optimum box volume or as small a one half the optimum box volume. The box tuning frequency will have to be changed to match the volume change. A 25% tuning ratio to box volume change is required. For example, if the optimum box volume is doubled, the optimum box tuning frequency must be lowered by 25 %. Conversely, if the box volume is increased by 50%, the tuning will have to be lowered by 12.5%. This works the same way in the other direction also. If the box volume is reduced by 50%, the optimum box tuning must be raised by 12.5%.

By using the design chart and identifying the optimum box size for your speaker, improved performance of the box can be achieved. For serious box designers, additional tools (i.e., computer speaker design programs) can be used to further improve the performance of any box. Either way, by using these simple design parameters, any box can be modified to insure getting the best performance possible.